Voltage generating circuits are widely used in semiconductor memory devices, SoC devices, and the like. In accordance with the progress of the micro-fabrication, low voltage operation and high integration of semiconductor elements, a voltage generating circuit is mounted together with such a device on a semiconductor chip to generate a voltage of a different level from that of a power supply voltage, which is supplied from the outside. The generated voltage, serving as an internal power supply voltage, is supplied to other circuits in the semiconductor chip.
Such a voltage generating circuit includes a step-down circuit that steps down the power supply voltage supplied from the outside, and a booster circuit that boosts the power supply voltage supplied from the outside.
As a step-down circuit, a voltage generating circuit such as a series regulator is known for use in a standby mode, for example, where current is less supplied. In addition, as another kind of step-down circuit, a source follower type voltage generating circuit is known. The source follower type voltage generating circuit is used in a mode for flowing current, for example, an active mode.
A source follower type voltage generating circuit is disclosed in Japanese Patent Application Publication No. 2003-178584 (Page 8, FIG. 10). The source follower type a voltage generating circuit is provided with an output transistor and a mirror transistor. The mirror transistor is of the same type as the output transistor, and is provided at the preceding stage of the output transistor.
The mirror transistor and the output transistor are supplied with a voltage of an outer power supply VDD. The outer power supply VDD is supplied to the drain of the output transistor. The gate of the mirror transistor is connected to the drain of the mirror transistor. The gates of the output transistor and the mirror transistor are connected to each other. The gate voltage of the mirror transistor is maintained at a constant level. The circuit structure allows the output transistor to output a stepped-down internal power supply voltage.
The source follower type voltage generating circuit is configured to equalize the internal power supply voltage stepped down by the output transistor with the source voltage of the mirror transistor.
However, as will be described below, the output transistor and the mirror transistor show different characteristics in the case the voltage of the outer power supply VDD is high. Accordingly, a problem arises that a difference occurs in current amount per unit width between the output transistor and the mirror transistor.
The phenomenon of showing the different characteristics will be specifically described with reference to FIG. 10.
FIG. 10 is load curves showing a relationship between a current value which flows through a source follower type voltage generating circuit and a gate-source voltage VGS of an output transistor and a mirror transistor. FIG. 10 shows the relationship when a voltage of an outer power supply VDD is high. A curve 10 shows a load characteristic of the mirror transistor. A curve 11 shows a load characteristic of the output transistor. In FIG. 10A, current value 20 shows the maximum load current value. A current value 21 shows the minimum load current value. A point 22 shows a current of the mirror transistor and a gate-source voltage VGS in a standby state respectively.
Thus, when the voltage of the voltage power supply VDD is high, a difference in load characteristics occurs between the mirror transistor and the output transistor, which outputs a voltage stepped-down from the voltage of the power supply VDD.
Accordingly, a relationship between a load current Ifk1 of the mirror transistor in a standby state and a load current Ifk2 of the output transistor to step down the voltage VDD is defined as follows:Ifk2>Ifk1  (1)
Moreover, in order to obtain a gate-source voltage VGS in a load current maximum region which is in an active state, a relationship between a load current Ifk1a of the mirror transistor and a load current Ifk2a of the output transistor that steps down the power supply voltage is defined as follows:Ifk2a>Ifk1a  (2)
Thus, a difference in the current amount per unit width occurs between the mirror transistor and the output transistor. The difference becomes remarkable with increase of a current amount of the voltage of the voltage power supply VDD.
As a result, the difference makes it difficult to maintain the source voltage of the mirror transistor at a predetermined value by controlling the gate voltage of the mirror transistor.